1. Field of the Invention
The present invention relates to engines and, particularly, to a small engine fuel injection system.
2. Description of Related Art
Small single and twin cylinder engines fueled by propane are in common use in such applications as floor buffing, commercial carpet cleaning and the like. Propane is used in these applications since it is inherently cleaner burning than gasoline, especially in an area of carbon monoxide exhaust emissions.
In such small engines, the propane fuel is typically metered to the engine either by a Venturi mixer or a variation of that principle, a spudded carburetor. Such systems are highly affected by small variations such as air cleaner restriction, regulator pressure drift, or fuel hose restriction. In addition these systems do not provide a consistent and predictable air/fuel mixture throughout the entire range of operating conditions. In addition, such systems do not allow for any monitoring of fuel system operation. Deteriorated engine tune, fuel composition deviation, altitude, or other conditions which could lead to undesirably high levels of CO (carbon monoxide) emission.
Closed loop fuel controls for large engines, in which feedback is provided concerning the actual air/fuel calibration, particularly for engines in motor vehicles, are known. In such modern fuel-injected engines, the primary input used to determine the amount of fuel required by the engine (i.e., determination of the pulse width for the fuel injectors) is a "speed density" reading. This is calculated using engine RPM (revolutions per minute) and the manifold absolute pressure (MAP) sensor voltage. The manifold absolute sensor voltage is a signal indicative of engine manifold pressure during intake and compression. In more advanced engines, additional sensor inputs such as intake air temperature, engine coolant temperature, and the like are used, but RPM and MAP are the principal inputs. In a multi-cylinder engine, the MAP voltage signal is fairly constant and thus is readily detectable, since the individual cylinder firings average out. For example, in an 8 cylinder engine, as illustrated in FIG. 1a, the MAP voltage signal is virtually a straight line, assuming constant throttle and engine load settings.
However, as can be seen in FIG. 1b, in single cylinder engines there is considerable pulsation and variation in the MAP voltage signal. Such extreme variation makes calculation of the proper fuel injector pulse width very difficult. One approach which has been attempted is to take the minimum and the maximum MAP voltages and average them to obtain a value to calculate fuel flow. However, this approach is suboptimal in achieving the proper fuel ratios.
Accordingly, there is a need for a closed loop fuel control system for small single and twin cylinder engines. There is similarly a need for a closed loop fuel injection system for a single cylinder engine which employs the MAP signal to achieve proper fuel ratio control.